This invention relates, in general, to strainers for removing liquid from fermentation reactors and more particularly to strainers for and methods for their use.
An example of such a fermentation system is U.S. Pat. No. 6,342,378, which shows an Anaerobic Phased Solids Digester System (APS-Digester) for producing biogas including hydrogen and methane, and is incorporated herein by reference.
In an APS-Digester System, organic solids are broken down and decomposed in fermentation reactors, such as a hydrolysis reactor, to produce a biogas. A liquid is produced as part of the fermentation reaction. As the process continues, the liquid needs to be removed from the reaction vessel.
In conventional systems, an agitator or pump pushes the liquid into a liquid draining assembly such as a drain hole and drain pipe. The outlet from the vessel is generally located at a lower region of the tank to avoid interfering with the fermentation reaction. With such conventional systems, the mixture pushed into the drain pipe contains liquid-suspended particles and the like which present several operational problems.
Such particulate matter sticks to surfaces and can build up over time. If the solid matter collects above a certain level, it can clog the vessel drain port and drain pipe. Even if a filter is used to prevent the passage of solid material into the drain pipe, the solid matter accumulates on the filter surface. In time, the entire liquid draining assembly must be purged and cleaned. This process requires shutting down the entire reactor which results in significant costs and operational inefficiencies.
Moreover, it is desirable to minimize the loss of the solid particle matter from the vessel through the drain. Particulate solid matter lost through the drain results in decreased efficiency. Because the system digests such matter, a decrease in the amount of solid matter—by expulsion through the drain pipe—leads to lower yields in the fermentation process. Therefore, it is desirable to remove the liquid while retaining any solid matter suspended in the liquid to optimize production of the biogas. The retained solids are preferably put back into the reactor for further decomposition until the particle is too small.
In conventional systems, strainer systems have been employed to address the need to remove liquid from vessels of fermentation reactors, such as anaerobic digesters, while retaining solids. These systems separate liquid from solids being digested in anaerobic digesters and at the same time drain liquid from the digesters. Exemplars of the prior art are strainer systems marketed by Tate Andale Company (Types 105-DS and 1051-DS) and ACME Engineering Products, Inc. (Models ACRS-OF and ACRS-L).
Such conventional strainers allow fluids containing particles to flow into and radially through a vertically oriented cylindrical screen, trapping the particles inside the screen. The trapped particles build-up on the inside surface of the screen and restrict the flow of clear fluids through the screen.
Over time, particles trapped in the strainer restrict the fluid flow out of the reactor vessel. When the flow restriction reaches a pre-determined value, the screen must be cleaned. Cleaning the screen requires shutting off the fluid source. A valve in the bottom of the inside screen cavity is then opened to allow the trapped particles to drop into a catchment container at the bottom of the strainer assembly. An individual then uses a scraper or paddle to sweep and clean the interior of the filter screen dislodging trapped particles that have collected on the inside surface of the screen and dropping them into the bottom catchment container. Once the inside surface of the screen is cleared of particles, the bottom valve is then closed and a drain valve in the lowest end of the bottom catchment container is opened. The operator then flushes fluid through the bottom catchment container (referred to as “blow-down”) to remove the collected residue from the system. Closing the bottom valve in the screen chamber and opening the main fluid shut-off valve allows the source fluid to enter once again into the interior of the screen and the filtration process resumes.
This strainer cleaning process periodically repeats with a frequency dependent upon the build-up of flow restriction through the strainer. In other systems, the screen cleaning process is automated by measuring the pressure drop across the filter screen with sensors that send signals to a controller. However, even in an automated system, solid particles trapped in the strainer must be flushed out of the system and therefore do not contribute to biogas production. Further, the cleaning system must still be performed periodically thereby requiring downtime in production.
Conventional strainers and fermentation systems further require periodic cleaning and mixing of the feed mixture in the reactor vessels. Because conventional strainers only provide for moving fluid into the strainer, the reactor vessels are prone to particulate build-up in and around the mouth of the strainer. Such systems also do not provide any means for mixing.
What is needed is a biogas production system and strainer which overcomes the above and other disadvantages of known systems. What is needed is a fermentation system for producing a biogas with decreased particulate build-up and waste. What is needed is an efficient and continuous strainer assembly. What is needed is a strainer assembly for cleaning solid particulate build-up and mixing the feed mixture.